1. Field of the Invention
The present invention is in the field of remote control systems for projectiles and missiles and more specifically lies in the area of providing on-board reference information to spin stabilized projectiles and missiles while in flight.
2. Description of the Prior Art
In the field of remote guided projectiles, which generally rely on spin stabilization over their flight trajectories, it is essential that the projectiles be instantaneously aware of the referential environment in which they are traveling, in order that received control (steering) information may be properly responded to by the projectile. Systems providing such reference information in the past have included: sun sensors; sky sensors; horizon sensors; gyroscopes; electrostatic field sensors; and electromagnetic field sensors in combination with ground station transmitted initialization information. Each of the above prior art systems have inherent deficiencies which limit universal acceptance in this field.
Systems which employ sun, sky, or horizon sensors are, of course, adversely affected by clouds, changing times of the day, launch location, flight direction and darkness. Therefore, such systems require sophisticated circuitry and skilled operators to set up the various parameters in order to obtain accurate and usable results.
Systems which employ gyroscopes are, of course, susceptible to mechanical damage due to pre-flight handling, high-G launch forces and flight vibrations. In addition, gyros generally require significantly large power sources and, therefore, present weight and space factors which must be accounted for with larger propulsion systems for a given payload.
Systems which employ electrostatic sensors, such as that disclosed in U.S. Pat. No. 3,990,659, have been developed which detect and generally provide unambiguous signal outputs, that are indicative of the gradient of the earth generated electrostatic field. The unambiguous signals provide a direct source by which the vertical plane and the "up" or "down" directions within that plane may be detected at any instant. However, under certain atmospheric anomalies, such as those which occur in thunderstorms, the electrostatic field gradient may become reversed in areas through which the flight path trajectory traverses. When such field reversal occurs, the electrostatic sensors will produce a signal that indicates an oppositely oriented reference direction to the guidance system of the projectile.
Systems which employ electromagnetic field sensors, such as that described in U.S. Pat. No. 3,860,199, have been developed which sense the electromagnetic field that the spinning missile is flying through. That system provides an ambiguous signal output from the on-board electromagnetic sensors and is continuously compared with a ground station electromagnetic field sensor signal. The ground station electromagnetic sensor signal is transmitted to the projectile in order that the on-board signal may be compared and a vertical reference direction may be determined on board the missile. Since electromagnetic field sensors produce signals that are inherently ambiguous (i.e., a defined directional reference cannot be determined by mere observance of the polarity of the signal), a rather complicated technique is disclosed in the aforementioned patent to initially determine the vertical "up" reference direction at the launch site and then optically transmit this information to the projectile rotating at an entirely different rate, which changes with time. The main disadvantage of such a system appears to be the rather complicated algorithm which must be performed both on board the projectile and at the launch site in order to achieve proper control. This, of course, is costly and may present reliability problems that cannot be tolerated in the field.